The invention relates to a method for producing a formoxysilane in accordance with the preamble to claim 1 or 2.
Acyloxysilane with the general formula RnSi[Oxe2x80x94C(O)Rxe2x80x2]4xe2x88x92n, in which R and Rxe2x80x2 represent an organic residue or hydrogen and n=0, 1, 2 or 3, form a class of compounds of high technical importance. The silylesters of the acetic acid (Rxe2x80x2=CH3xe2x80x94) are used on an industrial scale for the production of cross-linked RTV (room temperature vulcanization) silicones. The cross-linking of acyloxysilanes is started because the silylester group is easily hydrolyzed. With sealing materials, for example used for the pointing of joints in the sanitary region, the normal humidity in the air is sufficient to start a cross-linking (Ullman""s Encyclopedia of Industrial Chemistry, 4th ed., Vol. 22, 77-80 and 119-120). Formoxysilane (Rxe2x80x2=H) in addition has a potential for the production of defined SiO2 layers on various substrates, which is achieved through CVD (chemical vapor deposition) (EP 0778278 A).
A method of the aforementioned type follows from the Publication by H. Koinuma, F. Kawakami, H. Kato, H. Hirai, J. Chem. Soc. Chem. Comm., 1981, 213; and G. Sxc3xcss-Fink; J. Reiner, J. Organomet. Chem., 221, 1981, C36.
With this method, a silane is hydrolyzed with carbon dioxide in a solvent and in the presence of a homogeneous catalyst:
RRxe2x80x2Rxe2x80x3Sixe2x80x94H+Oxe2x95x90Cxe2x95x90Oxe2x86x92RRxe2x80x2Rxe2x80x3Sixe2x80x94Oxe2x80x94CHO
Complex compounds of palladium and ruthenium function as homogeneous catalyst. Up to 275 catalytic cycles per mol equivalent of the catalyst are achieved for the generating of formoxysilanes from the conversion of silanes of the type RRxe2x80x2Rxe2x80x3SiH (R, Rxe2x80x2, Rxe2x80x3=CH3xe2x80x94, C2H5xe2x80x94, xe2x80x94OCH3), wherein the reaction conditions are 60xc2x0 C. up to 120xc2x0 C. reaction temperature, 20 h to 60 h reaction time as well as 30 to 50 bar CO2 pressure. A re-processing and reuse of the catalysts is not mentioned.
Hydrolyzing carbon dioxide has the following advantages: carbon dioxide is non-toxic and is therefore quite harmless ecologically. In addition, it has no competition and is cheap as C1 substrate. Economic advantages can be achieved in particular if a more cost-effective catalyst is found, which can be recycled if necessary.
Thus, it is the object of the invention to find a simpler, easier to produce and thus most cost-effective catalyst for the aforementioned type of method, which can be obtained on a large scale and has a high activity and product selectivity. With a suitable realization of the method, the catalyst should be recyclable.
This object is solved with the catalyst specified in the characterizing section of claims 1 or 2. Preferred embodiments of the initially mentioned methods are listed in the additional claims.
Formoxysilanes with the general formula RnSi[Oxe2x80x94C(O)H]4xe2x88x92n, can be produced with the method according to the invention. R represents an organic or inorganic substituent, preferably an alkyl, alkenyl, aryl, aryloxy group or a halide or R1R2R3SiO residue, wherein R1, R2 and R3 is an additional organic or inorganic substituent. The index n can assume whole number values of 0 to 3, preferably 2 or 3.
According to the invention, ruthenium chloride (RuCl3xc2x7xH2O) is used in a first embodiment as catalyst. The starting material for this embodiment is an acyloxysilane in a solvent, to which the catalyst is added. The carbon dioxide is then pressed on. The reaction temperature should be in the range of 20xc2x0 C. to 120xc2x0 C. Pressures ranging form 1 bar to 200 bar are suitable for the carbon dioxide partial pressure and the reaction time is between 0.5 h and 50 h.
In a second embodiment of the invention, a compound of the transitional metal is used as the catalyst, which is formed through conversion of a halide of the transitional metal with a nitrile in the presence of the silane with formula RnSiH4xe2x88x92n. Thus, we are dealing with a pre-formed catalyst, consisting of a silane and one of the above-mentioned catalysts, preferably ruthenium chloride. The pre-forming preferably occurs without pressure. The silane and the catalyst, preferably RuCl3xc2x7xH2O can be dissolved for this in a solvent, for example acetone nitrile, and the solvent can be heated to reflux. A catalytically especially effective compound is generated during the pre-forming, which has the empirical formula C24H36Cl6N12Ru3.
The pre-formed catalyst has the advantage that the temperature for the subsequent hydrolyzing operation, carried out at the partial pressure specified for carbon, can be selected to be considerably lower than that for the first embodiment.
With the second embodiment, a solution of the silane in a solvent can thus be used as starting material, to which the catalyst, in particular with ruthenium chloride, is added at the specified temperature. Once the solution has been heated to reflux, the carbon dioxide is pressed on without further cleaning and the reactor is heated to a temperature of 30xc2x0 C. to 100xc2x0 C. while stirring the reactor content. The reaction time generally is between 1 and 40 hours. The reactor is subsequently cooled down, if necessary the pressure relieved, and the reaction product is isolated through distillation, rectification or extraction.
Suitable solvents are nitrites, benzenoid aromatic compounds, halogenated hydrocarbons or mixtures thereof Higher-boiling nitrites are preferably used as solvent for the lower-boiling products. The advantage is that the resulting products can be distilled directly out of the reaction mixture and the residue, which contains the catalyst, can be used for additional conversions.
Between 1/10 and 1/5000 equivalents of the catalyst are preferably used per equivalent Sixe2x80x94H-groups, referred to equivalents of the active metal.
The product yields with the formula Rnsi[Oxe2x80x94C(o)H]4xe2x88x92n in particular depend on the type of catalyst used and the substituents R of the silane. In the most favorable case, nearly quantitative yields are obtained, relative to the silane used. The TON (turnover number, or number of conversions per catalyst atom) reaches up to 5000 molacyloxysilane/molactive metal per batch with a TOF (turn over frequency, number of conversions per hour) of up to 500 hxe2x88x921. Multifunctional products (n=0, 1, 2) can be used as cross-linking components for silicon materials owing to their capacity to easily hydrolyze.